Wireless power transmission

ABSTRACT

Wireless power transmission methods, wireless power transmitters, wireless power receiving method, and wireless power receivers are disclosed. Wireless power is transmitted using power signal comprising a multi-sine waveform within a bandwidth to drive a plurality of antennas. Beam-forming coefficients are generated and the relative phases of the power signal used to drive the antennas is controlled by the beam-forming coefficients.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. National Phase Patentapplication Ser. No. 16/768,531 filed on May 29, 2020 under 35 U.S.C.371 of International Application No. PCT/SG2018/050587 filed on Nov. 30,2018, which claims the benefit of priority from Republic of SingaporePatent Application No. 10201710000U filed Dec. 1, 2017. The entiredisclosures of each of the above applications are incorporated herein byreference.

FIELD

The present disclosure relates to wireless power transmission. Inparticular, the present disclosure relates to wireless powertransmission which can be directed to one or more receivers.

BACKGROUND

Recent years have seen rapid growth in power and data networks relatedto the Industrial Internet of Things (IIoT). Such networks often requirenon-invasive sensor deployment in order to reduce the time fordeployment, down time of machines, wiring infrastructure cost and enableplacement of sensors in difficult-to-reach and hazardous areas. Oneoption for powering sensors in depolyments is batteries. However requirereplacement when discharged and they are corrosive.

In view of this a method of transmitting power to over relatively largedistances is required. Existing systems only allow for up to 5 meterscharging, with no radio frequency (RF) beam steering technology. Usingsuch systems for charging sensors over 25 meters distance will require avery high transmitter power that will not pass safety regulation.

SUMMARY OF THE INVENTION

According to a first aspect of the present disclosure, a wireless powertransmission method in a transmitter is provided. The wireless powertransmission method comprises: receiving feedback information from areceiver, the feedback information comprising indications of signalsreceived by antennas of the receiver from antennas of the transmitter;generating a power signal using the feedback information, the powersignal comprising a multi-sine waveform within a bandwidth; generatingbeamforming coefficients indicating relative phases for a plurality ofthe antennas of the transmitter using the feedback information; anddriving the plurality of antennas of the transmitter with the powersignal with relative phases of the plurality of antennas of thetransmitter controlled according to the beamforming coefficients.

The feedback information may comprise an indication of a charging rateof an energy storage device of the receiver.

The beamforming coefficients indicating relative phases for a pluralityof the antennas of the transmitter may be generated using the feedbackinformation comprises incrementally varying phase differences betweenthe signals transmitted on the plurality of antennas to optimize thecharging rate of the energy storage device of the receiver.

In an embodiment, the method further comprises determining coefficientsfor the multi-sine waveform that optimize the power received by thereceiver in a channel training procedure.

In an embodiment, the channel training procedure is initiated inresponse to a channel training initiation signal received from thereceiver, the channel training initiation signal indicating a drop in acharging rate of an energy storage device of the receiver.

In an embodiment, the method further comprises: receiving feedbackinformation from a plurality of receivers, wherein the feedbackinformation comprises an indication of signals received by antennas ofthe respective receiver from antennas of the transmitter and anindication of power usage or a stored power level at the respectivereceiver; determining a scheduling sequence for power transmission toeach of the receivers using the received indications of power usage atthe respective receivers; generating a power signal using the feedbackinformation, the power signal comprising a multi-sine waveform within abandwidth; generating beamforming coefficients indicating relativephases for a plurality of the antennas of the transmitter using thefeedback information; and driving the plurality of antennas of thetransmitter with the power signal with the relative phases of theplurality of antennas of the transmitter controlled according to thebeamforming coefficients with timing according to the schedulingsequence.

The method may further comprise selecting a frequency range as thebandwidth. The frequency range may be selected using the feedbackinformation. Alternatively, the frequency range is selected in responseto a user input.

According to a second aspect of the present disclosure a wireless powertransmitter is provided. The wireless power transmitter comprises: aplurality of power transmission antennas configured to wirelesslytransmit power to a receiver; a wireless communication module configuredto receive feedback information from the receiver, the feedbackinformation comprising indications of signals received by antennas ofthe receiver from the plurality of antennas of the transmitter; a signalgenerator configured to generate a power signal comprising a multi-sinewaveform within a bandwidth; a power signal optimizer configured tocontrol the signal generator using the feedback information; a phasecontrol logic configured to generate beamforming coefficients indicatingrelative phases for the plurality of the antennas of the transmitterusing the feedback information; a splitter configured to split the powersignal into a plurality of antenna signals each corresponding torespective antennas of the plurality of antennas; and a plurality ofphase shifters configured to shift the relative phase of the antennasignals according to the beamforming coefficients such that theplurality of antennas of the transmitter are driven with the powersignal with relative phases controlled according to the beamformingcoefficients.

In an embodiment the feedback information comprises an indication of acharging rate of an energy storage device of the receiver.

The phase control logic may be configured to generate the beamformingcoefficients by incrementally varying phase differences between thesignals transmitted on the plurality of antennas to optimize thecharging rate of the energy storage device of the receiver.

In an embodiment the wireless power transmitter further comprises achannel estimator configured to estimate channel state information forchannels between the plurality of antennas of the transmitter andantennas of the receiver and wherein the power signal optimizer isconfigured to determining coefficients for the multi-sine waveform thatoptimize the power received by the receiver.

The channel estimator may be configured to initiate a channel trainingprocedure in response to a channel training initiation signal receivedfrom the receiver, the channel training initiation signal indicating adrop in a charging rate of an energy storage device of the receiver.

The channel estimator may be configured to determine if the drop incharging rate was due to a scheduled change in target receiver and toinitiate the channel training procedure if the drop was not due to ascheduled change in target receiver.

In an embodiment the wireless power transmitter further comprises: asmart scheduling logic configured generate a scheduling sequence forpower transmission to each of a plurality of receivers using receivedindications of power usage at the respective receivers, wherein thesmart scheduling logic is configured to control the phase control logicconfigured to generate beamforming coefficients indicating relativephases for the plurality of the antennas of the transmitter to generatecoefficients with timing according to the scheduling sequence such thatthe power transmission antennas transmit power targeted at the pluralityof receivers according to the scheduling sequence.

In an embodiment the wireless power transmitter further comprises afrequency selector configured to select a frequency range as thebandwidth. The frequency range may be selected using the feedbackinformation. Alternatively, the frequency range may be selected inresponse to a user input.

According to a third aspect of the present disclosure, a feedback methodin a wireless power receiver is provided. The method comprises:receiving a wireless power transmission signal from a transmitter at anantenna or a plurality of antennas of the wireless power receiver;monitoring a charging rate in an energy storage device coupled to theantenna or plurality of antennas; identifying a change in the chargingrate; and sending a channel training initiation signal to thetransmitter.

According to a fourth aspect of the present disclosure, a wireless powerreceiver is provided. The wireless power receiver comprises: a pluralityof antennas configured to receive a wireless power transmission signalfrom a transmitter; an energy storage device coupled to the plurality ofantennas; a monitoring unit configured to monitor a charging rate of theenergy storage device; a channel estimator configured to identify a dropin the charging rate of the energy storage device and generate a channeltraining initiation signal; and a wireless communication moduleconfigured to send the channel training initiation signal to thetransmitter.

The wireless power receiver may comprise one or more RF-to-DC rectifierswhich convert the wireless power transmission signal from an RF signalto DC power.

In an embodiment, the channel estimator is further configured to causethe plurality of antennas to generate a pilot signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the present invention will be describedas non-limiting examples with reference to the accompanying drawings inwhich:

FIG. 1 shows a wireless power transmission system according to anembodiment of the present invention;

FIG. 2 is a block diagram showing a wireless power transmitter accordingto an embodiment of the present invention;

FIG. 3 is a block diagram showing a wireless power receiver according toan embodiment of the present invention;

FIG. 4 is a flowchart showing a method of wireless power transmissionaccording to an embodiment of the present invention;

FIG. 5 is a flowchart showing a method of wireless power transmission tomultiple wireless power receivers according to an embodiment of thepresent invention;

FIG. 6 is a flowchart showing a method of channel state estimation in awireless power transmission system according to an embodiment of thepresent invention;

FIG. 7 is a flowchart showing phase control in a wireless powertransmitter according to an embodiment of the present invention; and

FIG. 8 is a block diagram showing a hardware implementation of phasecontrol logic in a wireless power transmitter according to an embodimentof the present invention.

DETAILED DESCRIPTION

FIG. 1 is s block diagram showing a wireless power transmission systemaccording to an embodiment of the present invention. The wireless powertransmission system 100 may be part of a sensor network. The wirelesspower transmission system 100 comprises a wireless power transmitter 200and a plurality of wireless power receivers 300 a, 300 b and 300 c. Thewireless power transmitter 200 comprises a plurality of transmitterantennas 202 which can be controlled by the wireless power transmitter200 to transmit power over one or more radio frequency bands. Thefrequency may comprise conventional RF bands including 433 MHz, 915 MHz,2.4 GHz, 5.8 GHz, or millimeter-wave bands including 26, 28, 38, and 60GHz. The plurality of transmitter antennas 202 are controllable by thewireless power transmitter 200 to generate wireless power signals 206 a,206 b and 206 c directed towards respective wireless power receivers 300a, 300 b and 300 c. As will be described in more detail below, thewireless power transmitter 200 comprises control circuitry configured togenerate drive signals for the plurality of transmitter antennas 202 togenerated directed RF signals for each of the respective wireless powerreceivers 300 a, 300 b and 300 c.

The wireless power transmitter 200 further comprises a wirelesscommunication module 204. The wireless communication module 204 isconfigured to send and receive wireless information signals 208 a, 208 band 208 c from the wireless power receivers 300 a, 300 b and 300 c. Thewireless communication module 204 may be a Bluetooth module configuredto send and receive signals according to the Bluetooth standard.Alternatively, the wireless communication module 204 may be a configuredto send and receive signals according to the ZigBee, LoRa, WiFi, orNarrowband IoT (NB-IoT) communication protocols.

The wireless communication signals 208 a, 208 b and 208 c transmittedfrom the wireless power receivers 300 a, 300 b and 300 c comprisechannel state information indicating information on the signal strengthreceived by the respective wireless power receivers 300 a, 300 b and 300c from the respective wireless power signals 206 a, 206 b and 206 c.Alternatively or additionally, the wireless communication signals 208 a,208 b and 208 c transmitted from the wireless power receivers 300 a, 300b and 300 c may comprise information on a charging or discharging rateof an energy storage device such as a battery, capacitor, orsupercapacitor of the wireless power receiver 300 a, 300 b, or 300 c.The wireless communication signals 208 a, 208 b and 208 c may includecommands transmitted from the wireless power transmitter 200 to therespective wireless receivers 300 a, 300 b and 300 c. The commands maycomprise instructions to control sensors, which form part of thewireless receivers 300 a, 300 b and 300 c. The wireless communicationsignals 208 a, 208 b and 208 c transmitted from the wireless powerreceivers 300 a, 300 b and 300 c may also comprise indications of datasensed by sensors on the respective wireless power receivers 300 a, 300b and 300 c.

In some embodiments the wireless power signals 206 a, 206 b and 206 care transmitted according to a time division multiple access (TDMA)protocol to charge different wireless power receivers within a givenscheduling time horizon (such as a 24 hour period). The wireless powersignals 206 a, 206 b and 206 c and the wireless communication signals208 a, 208 b and 208 c may have different frequency bands or may sharethe same frequency band. When the wireless power signals 206 a, 206 band 206 c and the wireless communication signals 208 a, 208 b and 208 cshare the same frequency band interference between the wireless powersignals 206 a, 206 b and 206 c and the wireless communication signals208 a, 208 b and 208 c may be managed though a time division multipleaccess (TDMA) protocol. In such a protocol, the wireless powertransmission signals 206 a, 206 b and 206 c and the wirelesscommunication signals 208 a, 208 b and 208 c are implemented overorthogonal time slots.

FIG. 2 is a block diagram showing a wireless power transmitter accordingto an embodiment of the present invention. The wireless powertransmitter 200 shown in FIG. 2 corresponds to the wireless powertransmitter 200 shown in FIG. 1. The blocks and modules of the wirelesspower transmitter 200 may be implemented as specifically programmedhardware components or as software running on a microprocessor device oras a combination of the two.

The wireless power transmitter 200 comprises a user interface module 210which allows a user to input commands and selections. The user interfacemodule 210 enables a user to select a frequency band for wireless powertransfer from among a set of predefined bands, for example bandsincluding 433 MHz, 915 MHz, 2.4 GHz, 5.8 GHz, 26, 28, 38, or 60 GHz. Insome embodiments, the band for wireless power transmission may beselected automatically based on Channel State Information (CSI) for fullautonomy. Other bands can be included/excluded according to the hardwarespecifications and local authority limitations. The user interface 210also allows the user to communicate with the receiver via the wirelesscommunication module 204.

The wireless power transmitter 200 further comprises a frequencyselector 212. The frequency selector sets the center frequency of asignal generator 224 according to the power transmission frequency bandselected by the user via the user interface module 210. Alternatively,the frequency selector 212 may select the frequency automatically basedon the channel state information. The frequency selector 212 includes aRF switch connected to RF front-ends of different frequencies, where theswitch selects one RF front-end upon a time based on the given command.

The wireless power transmitter 200 comprises a sensor management unit214. The sensor management unit 214 is configured to extract informationrelated to sensors connected to the wireless power receivers 300 a, 300b and 300 c from the information exchanged over the wirelesscommunication channel through wireless communication signals 208 a, 208b and 208 c. The sensor management unit 214 stores this informationeither locally or on a cloud server, after which the end-user can accessit. Moreover, the sensor management unit 214 enables the end-userremotely to configure parameters of each sensor, e.g., its sampling rateor accuracy level. The wireless communication signals 208 a, 208 b and208 c are received by the wireless communication module 204. Thewireless communication signals 208 a, 208 b and 208 c fed back from thewireless power receivers 300 a, 300 b and 300 c may include anindication of the voltage across the Energy Storage System (ESS) at eachreceiver, and the current or power discharged/charged from/to the ESS.The ESS can be a battery, or a super-capacitor/capacitor. Suchinformation is needed for managing power delivery to multiple receivers,as explained in more detail below.

The wireless communication module 204 may be a Bluetooth module,alternative communication modules such as ZigBee, LoRa, WiFi, or NB-IoTcan be used instead of Bluetooth to realize the wireless feedbackmechanism.

The wireless power transmitter 200 comprises a smart scheduling logic(SSL) 216 which controls which wireless power receiver (or which set ofwireless power receivers) are scheduled to receive wireless powertransmission over time. Embodiments of the present invention adopt TDMAto deliver power to multiple receivers over different time slots withina certain scheduling time horizon, e.g., next 24 hours. At each timeslot, transmitter power signals are directed towards one receiver or acluster of receivers which are in close vicinity. Based on the currentand future power charging/discharging values of receivers' ESSs as wellas the state of charge (SoC) of ESSs, the SSL 216 prioritizes thesequence of power delivery to different receivers. Particularly, the SSL216 determines when and for how long each receiver needs to bewirelessly charged such that it has enough energy stored in its ESS tosmoothly run over the given scheduling time horizon. To achieve thisgoal, the SSL 216 solves the following optimization problem (OP1) inreal-time (or whenever needed):

$\begin{matrix}{{\underset{\{{P_{u}{(t)}}\}}{maximize}\mspace{14mu}{\int_{T}{\sum\limits_{u = 1}^{U}\;{{P_{u}(t)}\mspace{14mu}{dt}}}}}{{{{{Subject}\mspace{14mu}{to}\mspace{14mu}{SoC}_{{init},u}} + {P_{u}(t)} - {D_{u}(t)}} \geq {\underset{\_}{SoC}}_{u}},{u = 1},\ldots\;,U}{{p(t)}\overset{\Delta}{=}{\lbrack {{P_{1}(t)},{\ldots\mspace{14mu}{P_{U}(t)}}} \rbrack \in \Pi}}} & ( {{OP}\text{-}1} )\end{matrix}$

In OP1, the following notation is used:

Notation Definition U Number of receivers, indexed by u P_(u)(t)Deliverable wireless power to receiver u at time t (i.e., ESS powercharging value) D_(u)(t) Discharged power from the ESS of receiver u attime t (i.e., ESS power discharging value) SoC_(init,u) Initial state ofcharge of the ESS of receiver u SoC _(u) Minimum limit for the state ofcharge of the ESS of receiver u p(t) Vector of deliverable power to allreceivers Π Set of all possible power vectors

For p(t), the deliverable power to each receiver, which is also known asthe power charging value of its ESS, set by the power signal optimizer(PSO) 220 and the phase control logic (PCL) 222 which are describedbelow. For example, when the transmitter is beaming at receiver 1, byassuming that other receivers are sufficiently far from receiver 1, itfollows that p(t)=[P₁ 0, . . . , 0] with P₁ denoting the instantaneousamount of power delivered to receiver 1 (i.e., the instantaneous powercharging value of its ESS). Transmitter 1 feeds back the exact value ofP₁ to the transmitter via the existing feedback communication channel.

A channel estimator 218, estimates the CSI, including both channelamplitude and phase, between any pair of transmitter and (selected)receiver antennas using the feedback mechanism. The training processinvolves the wireless power receiver 300 transmitting a known pilotsignal to the wireless power transmitter 200. The channel estimator 218then analyses the received pilot signal to estimate the CSI. Thistechnique means that the CSI estimation calculation takes place on thewireless power transmitter 200 which reduces the processing load on thewireless power receiver 300. There are various CSI training techniquesbased on either least squares (LS) or minimum mean square error (MMSE).However, some embodiments use a novel mechanism to initiate CSI trainingin order to prevent unnecessary training and hence maximizing energyefficiency. This is described in more detail in FIG. 6.

A power signal optimizer (PSO) 220 controls the signal generator 224 togenerate a multi-sine waveform as the power signal, which includes Nsub-carriers (typically 8, 64, 256, 1024, or higher) with the totalbandwidth BW that can be adjusted according to the hardware constraintsand local authority rules and specifications (typically 100 KHz, 500KHz, or higher). The baseband multi-sine signal over time t is given as:

$\begin{matrix}{\sum\limits_{n = 1}^{N}\;{\sqrt{2}A_{n}\mspace{14mu}\cos\mspace{14mu}{( {2\pi\frac{BW}{N}n\mspace{14mu} t} ).}}} & (1)\end{matrix}$

The PSO 220 sets complex-valued amplitudes A_(n)=Â_(n)

θ_(n), n=1, . . . , N by solving an optimization problem (OP-2). Theobjective of (OP-2) is to maximize the peak of aggregated signalcollected by the receiver of our interests (i.e., the receiver selectedby SSL for power delivery at the current time slot), subject to thesignal generator's power budget, denoted by P_(max)≥0. By considering asingle receiver is considered for power delivery, receiver index u isomitted from the following equations.

$\begin{matrix}{{\underset{\{ A_{n}\}}{maximize}{{\sum\limits_{k = 1}^{K}\;{\sum\limits_{m = 1}^{M}\;{\sum\limits_{n = 1}^{N}\;{\sqrt{2}G_{agg}A_{n}h_{k,m,n}}}}}}}{{{Subject}\mspace{14mu}{to}\mspace{14mu}{\sum\limits_{n = 1}^{N}\; A_{n}^{2}}} \leq P_{\max}}} & ( {{OP}\text{-}2} )\end{matrix}$

Other practical constraints such as peak to average power ratio (PAPR)constraints for the transmit signal, i.e.,

${\frac{( {\sum\limits_{n = 1}^{N}\; A_{n}} )^{2}}{\sum\limits_{n = 1}^{N}\; A_{n}^{2}} \leq {PAPR}_{\max}},$

can be added to (OP-2) if necessary.

In (OP-2) the following notation is used:

Notation Definition A_(n) Amplitude of n-th sine wave, complex value KNumber of transmitter antennas, indexed by k M Number of receiverantennas, indexed by m h_(k,m,n) Channel between transmitter antenna kand receiver antenna m at n-th sub-carrier G_(agg) Aggregate gain fromtransmitter/receiver antennas and transmitter power amplifier |.|Absolute value operator

One method of solving (OP-2) is as follows:

Let h_(n)

Σ_(k=1) ^(K)Σ_(m=1) ^(M)h_(k,m,n), where h_(n) is in general acomplex-valued number and can be represented as h_(n)=ĥ_(n)

ψ_(n). The solution to (OP-2) is set as θ_(n)=ψ_(n), n=1, . . . , N,with:

$\begin{matrix}{{{\hat{A}}_{n} = {{\hat{h}}_{n}\sqrt{\frac{P_{\max}}{\sum\limits_{n = 1}^{N}\;{\hat{h}}_{n}^{2}}}}},{n = 1},\ldots\;,{N.}} & (2)\end{matrix}$

In (OP-2), the number of sub-carriers N is set according to thestructure of the RF-to-DC rectifier used in the considered receiver.Generally, larger N is needed when the forward voltage of the diode(s)or transistor(s) of the RF-to-DC rectifier of the wireless powerreceiver is larger (e.g., 0.7V). However, the maximum value for N islimited by the PAPR at the transmitter. Hence, N can be set according tothe data available from the system while it can be updated in (OP-2)whenever necessary.

A second, sub-optimal solution to (OP-2) is given as equal-powerallocation with zero phase,

${A_{n} = {\sqrt{\frac{P_{\max}}{N}}{\measuredangle 0}}},$

n=1, . . . , N. This solution can be implemented for the case that CSIis not available or when the transmitter does not have sufficientcomputational resources to solve (OP-2) directly. A performancereduction as compared to the first solution is expected.

The above two solutions are implemented over the signal generator 224,which can be a Software Defined Radio (SDR). An entry-level SDR issufficient to implement the multi-sine waveform when the allocatedbandwidth is sufficiently small (typically below 1 MHz).

The output of the smart scheduling logic (SSL) 216 is used by a phasecontrol logic (PCL) 222 to steer the power signal towards the receiverselected by SSL 216 at the current time slot.

The signal generator 224 uses a DC supply 226 to generate the basebandmulti-sine signal. This signal is amplified by a power amplifier (PA)228 and split into K signals by a splitter 230. The phase of each of theK signals is adjusted according to the output of the SSL 216 by phasecontrollers 232 a to 232K. The resulting signals 234 a to 234K are usedto drive respective antennas 202 a to 202K of the wireless powertransmitter. Thus the antennas 202 a to 202K produce a wireless powersignal 206.

In some embodiments the charging power value of the receiver's ESS isused to adjust the phase of each transmitter antennas 202 a to 202K.Alternatively, the change in the voltage across the ESS can be used.Under fixed load power consumption at the receiver, when the voltageacross its ESS increases (decreases) it shows its charging power valuealso increases (decreases). Such information is shared via the existingwireless feedback channel. The process is explained in the following.

To achieve RF beam-steering, phases of signals 234 a to 234K drivingindividual transmitter antennas are being adjusted with phasecontrollers 232 a to 232K such that the deliverable power to theconsidered receiver is maximized, i.e., the charging power value of itsESS is maximized. As a starting point, the PCL 222 sets the phase ofeach transmitter antenna k, k∈{1, . . . , K}, as with

${\phi_{k} = {\phi_{k,{init}}\overset{\Delta}{=}\frac{\sum\limits_{m = 1}^{M}\;{\sum\limits_{n = 1}^{N}\;\psi_{k,m,n}}}{M \times N}}},$

with ψ_(k,m,n) denoting the channel phase between transmitter antenna kand receiver antenna m at n-th sub-carrier (if CSI is not available,then ϕ_(k,init)=0). With such an initial point, phases ϕ_(k) aregradually tuned as described with reference to the Flowchart shown inFIG. 7, with Δϕ (typically

$ {{{\Delta\phi}} \leq \frac{\pi}{2}} )$

denoting the step size for phase tuning. The above procedure may berepeated for multiple cycles to achieve the best beam-steering result.

In some embodiments, at each cycle of searching, Δϕ decreasesprogressively to enhance the accuracy of the search. The obtained phaseshift values can be saved in a look-up table to be used as an initialpoint to direct power signal towards the same receiver in the future.

The PCL 222 is capable of steering independent power signal beams to twoor more receivers concurrently, but the wireless power transmitter 200needs to have a large number of antennas spaced sufficiently far fromeach other (applicable to frequency bands of above 5.8 GHz whichantennas are smaller). To achieve this goal, the transmitter antennasare firstly divided into two or more subsets, where each subset isassigned to steer a beam to one receiver.

FIG. 3 is a block diagram showing a wireless power receiver according toan embodiment of the present invention. The wireless power receiver 300comprises a plurality of antennas 302 that are arranged to receive awireless power signal 206 from the wireless power transmitter 200. Thewireless power receiver 300 further comprises a wireless communicationmodule 304 that allows communication over a wireless communicationprotocol with the wireless communication module 204 of the wirelesspower transmitter 200. As described above, the wireless communicationmay be a Bluetooth protocol, alternative communication protocols such asZigBee, LoRa, WiFi, or IoT-NB can be used instead of Bluetooth torealize the wireless feedback mechanism. The blocks and modules of thewireless power receiver 300 may be implemented as specificallyprogrammed hardware components or as software running on amicroprocessor device or as a combination of the two.

The wireless power receiver 300 comprises a sensor device 306. Thesensor device 306 can be any type of sensor and sensor data captured bythe sensor device 306 is transmitted to the wireless power transmitter200 via the wireless communication module 304.

The sensor device 306 is powered by an energy storage system (ESS) 310.The ESS can be a battery, or a super-capacitor/capacitor. The ESS 310supplies power to the wireless power receiver 300 and is charged bypower captured by the plurality of antennas 306 from the wireless powersignal 206 transmitted by the wireless power transmitter 300.

The plurality of antennas 306 are coupled to an RF-to-DC rectifier 312which converts the energy captured by the antennas 306 into a DCvoltage. The RF-to-DC rectifier 312 is coupled to a DC-to-DC convertor314 which converts the DC voltage output of the RF-to-DC rectifier 312to a suitable voltage for charging the ESS 310.

A receiver performance monitoring unit (RPMU) 316 is coupled to the ESS310. The RPMU 316 comprises a power measurement unit to monitor thepower charging/discharging values of the ESS 310 in real time. The RPMU316 is also configured to determine the State-of-Charge (SoC) of the ESS310. This can be implemented by monitoring the Open Circuit Voltage(OCV) of the battery/supercapacitor/capacitor and then comparing it witha stored OCV-versus-SoC chart to estimate SoC. The voltage andcharging/discharging current of the ESS 310 can be measured via separatesensors or a single power management IC, like INA 220 from TexasInstruments, can be used to monitor both parameters.

The wireless power receiver 300 further comprises a channel estimator318. The channel estimator 318 is paired to the channel estimator 218 ofthe wireless power transmitter 200. The channel estimator 218 of thewireless power receiver 300 stores a set of pre-defined pilot signals.During a channel estimation process, these pilot signals are transmittedby the plurality of antennas 302 of wireless power receiver 300 to theplurality of antennas 202 of the wireless power transmitter 200. Thechannel estimator 218 of the wireless power transmitter 200 uses thereceived pilot signals to estimate CSI of channels between the antennas302 of the wireless power receiver 300 and the antennas 202 of thewireless power transmitter 200. The channel estimator 218 of thewireless power transmitter may use Maximum Likelihood (ML), leastsquares (LS) or minimum mean square error (MMSE) to find the CSI.

FIG. 4 is a flowchart showing a method of wireless power transmissionaccording to an embodiment of the present invention. The method 400shown in FIG. 4 is implemented on the wireless power transmitter 200shown in FIG. 2.

In step 402, the wireless communication module 204 of the wireless powertransmitter 200 receives a feedback signal from the wireless powerreceiver 300. The feedback signal comprises indications of power signalsreceived by antennas 302 of the wireless power receiver 300. Thefeedback signal may indicate the charging rate of the energy storagedevice 310 of the wireless power receiver 300.

In step 404, the power signal optimizer 220 of the wireless powertransmitter 200 generates an indication of a power signal. The powersignal is a multi-sine waveform and the power signal optimizerdetermines optimized amplitudes for subcarriers of the multi-sinewaveform using the feedback information.

In step 406, the phase control logic 222 of the wireless powertransmitter 200 generates beam-forming coefficients which indicaterelative phases for the antennas 202 of the wireless power transmitter200.

In step 408, the antennas 202 of the wireless power transmitter 200 aredriven according to the beam-forming coefficients. The power signaloptimizer 220 controls the signal generator 224 to generate the powersignal. This power signal is then amplified by the power amplifier 228and split into a signal for each antenna by the splitter 230. The phasesof the signals for each of the antennas 202 are modified by phaseshifters 232 according to the beam-forming coefficients. Then the phaseshifted power signals are used to drive the respective antennas 202.

FIG. 5 is a flowchart showing a method of wireless power transmission tomultiple wireless power receivers according to an embodiment of thepresent invention. The method 500 shown in FIG. 5 is implemented on thewireless power transmitter 200 shown in FIG. 2.

In step 502, the wireless communication module 204 of the wireless powertransmitter 200 receives a feedback signal from the each of the wirelesspower receivers 300. The feedback comprises indications of power signalsreceived by antennas 302 of the each of a plurality of wireless powerreceivers 300. The feedback also comprises indications of a power usageor stored power level at each of the wireless power receivers 300.

In step 504, the smart sequencing logic 216 of the wireless powertransmitter 200 determines a scheduling sequence for power transmissionto each of the wireless power receivers using the indication of thepower usage or stored power level at each of the wireless powerreceivers 300.

In step 506, the power signal optimizer 220 of the wireless powertransmitter 200 generates an indication of a power signal. The powersignal is a multi-sine waveform and the power signal optimizerdetermines optimized amplitudes for subcarriers of the multi-sinewaveform using the feedback information.

In step 508, the phase control logic 222 of the wireless powertransmitter 200 generates beam-forming coefficients which indicaterelative phases for the antennas 202 of the wireless power transmitter200.

In step 510, the antennas 202 of the wireless power transmitter 200 aredriven according to the beam-forming coefficients with timing accordingto the scheduling sequence. The power signal optimizer 220 controls thesignal generator 224 to generate the power signal. This power signal isthen amplified by the power amplifier 228 and split into a signal foreach antenna by the splitter 230. The phases of the signals for each ofthe antennas 202 are modified by phase shifters 232 according to thebeam-forming coefficients. The smart scheduling logic 216 controls thephase control logic 222 to generate the beam-forming coefficientscorresponding to each of the wireless power receivers according to thescheduling sequence. Then the phase shifted power signals are used todrive the respective antennas 202.

FIG. 6 is a flowchart showing a method of channel state estimation in awireless power transmission system according to an embodiment of thepresent invention.

As shown in FIG. 6 the method 600 of channel state estimation comprisesa channel training procedure 620. The channel training procedure 620 isinitiated when the wireless power transmitter 200 starts the process ofsteering the power signal towards a receiver of interest at thebeginning of power transmission 602. Subsequently, the channel trainingprocedure 620 is started when the power charging value of the receiver'sESS suddenly reduces by more than a given percentage, set 20% bydefault. In this case, in step 610, a drop in ESS power charging valueis identified. In step 612 the drop is compared with a threshold (forexample 20%). If the drop is greater than the threshold, the wirelesspower transmitter 200 validates the request. The request is validated instep 614. To validate the request, the wireless power transmitter 200determines whether the time slot allocated for delivering power thisreceiver has passed over or not. If the time slot is over, it means thatthe wireless power transmission towards this receiver is stoppedintentionally, and no channel estimation is needed. Hence, the requestis invalid, and a signal will send the receiver to not send any furtherrequest for channel re-training until the next session of charging, inthis case, the method moves to step 616 and terminates. On the otherhand, if the time slot is still not over, the sudden drop in the powercharging value is mostly due to the outdated CSI (e.g., the receiver ismoved and/or surrounding scatterers are moving). Hence, the request isvalid and the method moves to the channel training procedure 620. Thechannel estimator 318 of the wireless power receiver 200 may carry outsteps 610 and 612, namely identifying that there has been a drop in theESS power charging value. A message may then be sent via the wirelesscommunication module 304 of the wireless power receiver 300 to thewireless power transmitter 200, Upon receipt of this message, thechannel estimator 218 of the wireless power transmitter 200 thendetermines in step 614 whether the drop was due to a scheduled change intransmission.

The channel training procedure 620 comprises the following steps.Initially, a timer at each of the wireless power transmitter 200 and thewireless power receiver 300 is synchronized in step 621. In step 622,the wireless power receiver 300 sends the pre-define pilot signal to thewireless power transmitter 200. The wireless power transmitter 200detects the pilot signal in step 623. In step 624, the channel estimator218 of the wireless power transmitter 200 uses the received pilot signaland a stored indication of the pilot signal to calculate channel CSI,explained in the following:

The channel estimator 218 of the wireless power transmitter 200 can useMaximum Likelihood (ML), least squares (LS) or minimum mean square error(MMSE) to find the CSI. CSI between transmit antenna k and receiverantenna m, is estimated as follows. The wireless power receiver 300 willsend the following Q signals

${\sum\limits_{n = 1}^{N}\;{A_{n,T_{1}}\mspace{14mu}\cos\mspace{14mu}( {2\pi\frac{BW}{N}n\mspace{14mu} t} )}},{\sum\limits_{n = 1}^{N}\;{A_{n,T_{2}}\mspace{14mu}\cos\mspace{14mu}( {2\pi\frac{BW}{N}n\mspace{14mu} t} )}},\ldots\;,{\sum\limits_{n = 1}^{N}\;{A_{n,T_{Q}}\mspace{14mu}\cos\mspace{14mu}( {2\pi\frac{BW}{N}n\mspace{14mu} t} )}}$

in a given sequences. The complex valued constants [A_(n,T) ₁ ,A_(n,T) ₂, . . . , A_(n,T) _(Q) ] are all known to the wireless power transmitter200. Let represent signals received at the transmitter antenna k as

${\sum\limits_{n = 1}^{N}\;{{\overset{\sim}{A}}_{n,T_{1}}^{k}\mspace{14mu}\cos\mspace{14mu}( {2\pi\frac{BW}{N}n\mspace{14mu} t} )}},{\sum\limits_{n = 1}^{N}\;{{\overset{\sim}{A}}_{n,T_{2}}^{k}\mspace{14mu}\cos\mspace{14mu}( {2\pi\frac{BW}{N}n\mspace{14mu} t} )}},\ldots\;,{\sum\limits_{n = 1}^{N}\;{{\overset{\sim}{A}}_{n,T_{Q}}^{k}\mspace{14mu}\cos\mspace{14mu}{( {2\pi\frac{BW}{N}n\mspace{14mu} t} ).}}}$

Let h_(k,m)=[k_(k,m,1), . . . , h_(k,m,N)] denote the CSI vector betweentransmit antenna k and receiver antenna m. The CSI is estimated using MLas follows:

$h_{k,m}^{ML} = {\arg\mspace{14mu}{\min\limits_{h_{k,m}}\mspace{14mu}{\sum\limits_{n = 1}^{N}\;{\sum\limits_{q = 1}^{Q}\;( {{\overset{\sim}{A}}_{n,T_{q}}^{k} - {h_{k,m,n}A_{n,T_{q}}}} )^{2}}}}}$

In step 630, the CSI is updated with the calculated values and thechannel estimator 318 of the wireless power receiver 300 monitors theESS power charging value to determine whether a drop occurs in whichcase the method moves to step 610.

FIG. 7 is a flowchart showing phase control in a wireless powertransmitter according to an embodiment of the present invention. Themethod 700 shown in FIG. 7 is carried out by the phase control logic(PCL) 222 of the wireless power transmitter 200. To do so, in the PCL222 executes the method shown in FIG. 7. In step 702, the phase of eachtransmitter antenna k, k∈{1, . . . , K}, is set as

${\phi_{k} = {\phi_{k,{init}}\overset{\Delta}{=}\frac{\sum\limits_{m = 1}^{M}\;{\sum\limits_{n = 1}^{N}\;\psi_{k,m,n}}}{M \times N}}},$

with ψ_(k,m,n) denoting the channel phase between transmitter antenna kand receiver antenna m at n-th sub-carrier (if CSI is not available,then ϕ_(k,int)=0).

Then the method initializes the phase of the first antenna by settingk=1 in step 704. The phase ϕ_(k) is then gradually tuned by incrementingby Δϕ (typically

$ {{{\Delta\phi}} \leq \frac{\pi}{2}} )$

in step 706. Then, in step 708 it is determined if the incrementalincrease in the phase results in an increase in power delivered. If theincremental increase in phase does result in an increase in powerdelivered, the method returns to step 706 and the phase is incrementedagain.

If the incremental increase in phase does not increase the powerdelivered, then the method moves to step 710 in which the previousincremental increase in phase is reversed and the phase for the kthantenna is set as the phase ϕ_(k) prior to the increase. Then in step712 it is determined whether the phase for all antennas has been set. Ifthe phase for all antennas has been set (i.e. if k=K) then the methodends in step 716. Otherwise, k is incremented in step 714 and the methodis repeated for the kth antenna. The above procedure may be repeated formultiple cycles to achieve the best beam-steering result. At each cycleof searching, Δϕ decreases progressively to enhance the accuracy of thesearch.

The obtained phase shift values can be saved in a look-up table to beused as an initial point to steer the power signal towards the samereceiver in the future.

The PCL 222 is capable of finding a single power signal beam which aimsto maximize the weighted deliverable power to two or more receivers atthe same time. Thus, in some embodiments, the question of step 708 isreplaced by “Does the weighted charging power value of targetedreceivers increase”. For example, let P₁ and P₂ denote the instantaneouspower delivered to receivers 1 and 2, respectively. In this case, theweighted power charging value of targeted receivers can be defined asw₁P₁+w₂P₂, where w₁ and w₂ are constants defined by the user. In thiscase, at each iteration, the change in w₁P₁+w₂P₂ is being monitored.Other objectives can be used as the decision metric in the PCL 222.

FIG. 8 is a block diagram showing one possible hardware implementationof phase control logic in a wireless power transmitter according to anembodiment of the present invention.

In the embodiment shown in FIG. 8, analog phase shifters 810 are used toimplement the phase control logic 222 and control the phase of thesignals for each of the antennas. One analog phase shifter 810 is usedper transmitter antenna. In this embodiment, an analog phase shiftersuch as SPHSA-152+ from Mini-Circuits Inc., is used. The phase shiftvalue for SPHSA-152+ is a function of its input voltage. As shown inFIG. 8, the input voltage pins 812 of the phase shifter 810 can be usedto control the phase shift. To control the input voltage, adigital-to-analog converter (DAC) 820 is used, e.g., MCP4725 fromMicrochip Technology. A microcontroller 830 which runs the PCL 222algorithm sends digital commands 832 to the DAC 820 to get the desiredtarget voltage 822 across the input voltage pins 812 of the phaseshifter 810. The proposed digitally-controlled analog phase shiftcontrol topology is of high accuracy. Specifically, with MCP4725 used asDAC, 4096 different phase values can be realized. Moreover, the proposedtopology is scalable to a large number of transmitter antennas due tothe fact that MCP4725 supports High-Speed I2C protocol and hasnon-volatile memory (EEPROM). Thus, multiple DACs can be controlled viaa single microcontroller seamlessly.

As shown in FIG. 8, the microcontroller 830 receives feedbackinformation 834 indicating the charging power of the ESS of the receiverbeing targeted and generates digital commands to vary the phase. Thephase shifter 810 associated with the kth channel receives the RF inputRFin from the splitter 230 and generates the phase shifted output signalRFout which is used to drive the kth antenna.

The embodiment described above with reference to FIG. 8 is particularlysuited to sub-GHz systems. For high-frequency radio systems like 5.8 GHzand above, digital phase shifters are used to implement the PCL, forwhich a microcontroller (or FPGA, or Raspberry Pi, etc.) sends digitalcommands to each phase shifter to set its phase shift value directly.

After each phase shifter, a Power Amplifier (PA) may be used to boostthe signal strength going to the antenna. Moreover, the power of PAs canbe controlled using a similar procedure shown in FIG. 7 to enhance thebeam-steering accuracy and reduce sidelobes.

Whilst the foregoing description has described exemplary embodiments, itwill be understood by those skilled in the art that many variations ofthe embodiments can be made within the scope and spirit of the presentinvention.

1. A wireless power transmission method, in a transmitter, the methodcomprising: receiving feedback information from a receiver, the feedbackinformation comprising indications of signals received by antennas ofthe receiver from antennas of the transmitter; generating a power signalusing the feedback information, the power signal comprising a multi-sinewaveform within a bandwidth; generating beamforming coefficientsindicating relative phases for a plurality of the antennas of thetransmitter using the feedback information; and driving the plurality ofantennas of the transmitter with the power signal with relative phasesof the plurality of antennas of the transmitter controlled according tothe beamforming coefficients.
 2. A wireless power transmission methodaccording to claim 1, wherein the feedback information comprises anindication of a charging rate of an energy storage device of thereceiver.
 3. A method according to claim 2, wherein generatingbeamforming coefficients indicating relative phases for a plurality ofthe antennas of the transmitter using the feedback information comprisesincrementally varying phase differences between the signals transmittedon the plurality of antennas to optimize the charging rate of the energystorage device of the receiver.
 4. A method according to claim 1,further comprising determining coefficients for the multi-sine waveformthat optimize the power received by the receiver in a channel trainingprocedure.
 5. A method according to claim 4, wherein the channeltraining procedure is initiated in response to a channel traininginitiation signal received from the receiver, the channel traininginitiation signal indicating a drop in a charging rate of an energystorage device of the receiver.
 6. A wireless power transmission methodaccording to claim 1, further comprising selecting a frequency range asthe bandwidth.
 7. A wireless power transmission method according toclaim 6, wherein the frequency range is selected using the feedbackinformation.
 8. A wireless power transmission method according to claim6, wherein the frequency range is selected in response to a user input.9. A wireless power transmitter comprising: a plurality of powertransmission antennas configured to wirelessly transmit power to areceiver; a wireless communication module configured to receive feedbackinformation from the receiver, the feedback information comprisingindications of signals received by antennas of the receiver from theplurality of antennas of the transmitter; a signal generator configuredto generate a power signal comprising a multi-sine waveform within abandwidth; a power signal optimizer configured to control the signalgenerator using the feedback information; a phase control logicconfigured to generate beamforming coefficients indicating relativephases for the plurality of the antennas of the transmitter using thefeedback information; a splitter configured to split the power signalinto a plurality of antenna signals each corresponding to respectiveantennas of the plurality of antennas; and a plurality of phase shiftersconfigured to shift the relative phase of the antenna signals accordingto the beamforming coefficients such that the plurality of antennas ofthe transmitter are driven with the power signal with relative phasescontrolled according to the beamforming coefficients.
 10. A wirelesspower transmitter according to claim 9, wherein the feedback informationcomprises an indication of a charging rate of an energy storage deviceof the receiver.
 11. A wireless power transmitter according to claim 10,wherein the phase control logic is configured to generate thebeamforming coefficients by incrementally varying phase differencesbetween the signals transmitted on the plurality of antennas to optimizethe charging rate of the energy storage device of the receiver.
 12. Awireless power transmitter according to claim 9, further comprising achannel estimator configured to estimate channel state information forchannels between the plurality of antennas of the transmitter andantennas of the receiver and wherein the power signal optimizer isconfigured to determining coefficients for the multi-sine waveform thatoptimize the power received by the receiver.
 13. A wireless powertransmitter according to claim 12, wherein the channel estimator isconfigured to initiate a channel training procedure in response to achannel training initiation signal received from the receiver, thechannel training initiation signal indicating a drop in a charging rateof an energy storage device of the receiver.
 14. A wireless powertransmitter according to claim 13, wherein the channel estimator isconfigured to determine if the drop in charging rate was due to ascheduled change in target receiver and to initiate the channel trainingprocedure if the drop was not due to a scheduled change in targetreceiver.
 15. A wireless power transmitter according to claim 9, furthercomprising a frequency selector configured to select a frequency rangeas the bandwidth.
 16. A wireless power transmitter according to claim15, wherein the frequency range is selected using the feedbackinformation.
 17. A wireless power transmitter according to claim 15,wherein the frequency range is selected in response to a user input. 18.A feedback method in a wireless power receiver, the method comprising:receiving a wireless power transmission signal from a transmitter at anantenna or a plurality of antennas of the wireless power receiver;monitoring a charging rate in an energy storage device coupled to theantenna or plurality of antennas; identifying a change in the chargingrate; and sending a channel training initiation signal to thetransmitter.
 19. A wireless power receiver comprising a plurality ofantennas configured to receive a wireless power transmission signal froma transmitter; an energy storage device coupled to the plurality ofantennas; a monitoring unit configured to monitor a charging rate of theenergy storage device; a channel estimator configured to identify a dropin the charging rate of the energy storage device and generate a channeltraining initiation signal; and a wireless communication moduleconfigured to send the channel training initiation signal to thetransmitter.
 20. A wireless power receiver according to claim 19,wherein the channel estimator is further configured to cause theplurality of antennas to generate a pilot signal.